In a stratified charge spark ignition engine, steps are taken intentionally to ensure that the combustible charge is not homogeneous. The fuel is concentrated in one region of the charge while other regions have high concentration of air or EGR (exhaust gas recirculation) gases, depending on the design of the intake system.
This stratification may take place in different ways within the combustion chamber. The common forms of stratification are axial stratification in which the fuel concentrated at the top of the combustion chamber and radial stratification in which the fuel is concentrated near the central axis of the combustion chamber.
Other forms of stratification are created by causing the intake charge to tumble, that is to say rotate about an axis perpendicular to the cylinder axis. Such motion is not to be confused with swirl in which gases rotate about the axis of the cylinder, the latter normally resulting in radial stratification.
When tumble is promoted in the intake charge, stratification can occur in one of two ways. In one way, if the composition of the gases varies across the width of the combustion chamber, a sandwich-like structure with vertical layers is created that is referred to as vertical stratification, the vertical direction being taken as the axis of reciprocation of the piston.
The present invention is concerned with the other form of stratification produced by tumble, which is described in copending Patent Appln. No. 9505894.7 filed on Mar. 23, 1995 and is termed envelope stratification in that application. Here, the tumble produces a horizontal cylinder of one composition enveloped in a second outer layer having a different composition, the two layers tumbling in unison at right angles to the cylinder axis.
In order to produce radial stratification, the gases entering the combustion chamber can be split into two streams of different composition with the separation between them extending parallel to the stem of the intake valve. By aiming the gases tangentially into the combustion chamber, one stream can be directed to swirl near the cylinder wall while the other stream is directed towards the center of the cylinder. Envelope stratification, on the other hand, is produced by splitting the intake gases into two streams of different composition with the separation between them extending at right angles to the stem of the intake valve. The lower of the two streams is directed towards the center of the combustion chamber while the upper stream is directed towards the roof of the combustion chamber, the two streams then tumbling together at right angles to the cylinder axis.
Dividing an intake port by a partition extending at right angle to the valve stem presents a problem. Fuel needs to be introduced into the lower channel and should ideally be aimed at the intake valve. However, the presence of the partition hinders the line of sight access to the valve and it is not possible to position the fuel injector sufficiently near the intake valve in such a manner as to avoid wall wetting.
U.S. Pat. No. 4,308,829 describes an intake system for an internal combustion engine, having a cylinder head, an intake port in the cylinder head controlled by an intake poppet valve having a valve stem, and a twisted partition wall in the intake port dividing the intake port into two helical channels. The twisted partition wall in this patent is designed to promote swirl and its purpose is to make the gas stream rotate about the axis of the valve stem.